Mycoplasma bovis - stealthy or opportunistic?

Mycoplasma bovis infections permeate all segments of the cattle industry from calves to adult cows, dairy to beef and veal operations to feedlots. “People in a dairy context have a different definition of Mycoplasma problems from people in a beef context,” says Ricardo Rosenbusch, DVM, PhD, a Mycoplasma researcher at Iowa State University. “In a dairy context, more and more people are aware of the variety of Mycoplasmas involved in producing diseases. In the beef context, a Mycoplasma problem is Mycoplasma bovis.”

Mycoplasmas are bacteria that have no protective cell walls and minimal genetic material. They often elude their host’s immune systems to survive for long periods of time. The bacteria colonize the respiratory tract and other mucosal surfaces and infect the bloodstream. Mycoplasma bovis has been associated with a variety of bovine diseases, such as abortion and low fertility, arthritis, keratoconjunctivitis, mastitis, pneumonia and synovitis. This series will focus on Mycoplasma bovis and its contribution to otitis, arthritis and pneumonia. (For information on Mycoplasma mastitis, see Bovine Veterinarian September and October 2001.)

Not all Mycoplasmas are as important economically as M. bovis. “The dairy industry is concerned, for example, with a chronic cough in 2- and 3-month-old dairy calves,” Rosenbusch adds. “That could be a Mycoplasma dispar. That’s a very frequent type of presentation, but it’s not a medical emergency.”

Mycoplasma bovis infections in calves, stockers and feedlot cattle can run the gamut from ear infections to arthritis and pneumonia.

An “all-purpose pathogen,” Mycoplasma organisms can attach to the mucosal layer of the respiratory epithelium and reside in the respiratory tract as an opportunist and work synergistically with other pathogens in respiratory disease. Because the organism has an affinity for membrane surfaces, it can readily colonize the synovia in one or several joints and lead to septic joints and arthritis problems. In calves and even lightweight stockers, ear infections due to Mycoplasma bovis can cause ear droop, excessive discharge from the eye, facial paralysis, head tilt and purulent material in the external ear canal.

TransmissionIt’s believed Mycoplasma bovis can be transmitted by a variety of ways. Rosenbusch says it can be transmitted through the udder to a calf. “We’ve had a situation where the only way we could explain how Mycoplasma bovis got into a clean herd was the purchase of a pregnant heifer that must have transmitted, through colostrum, to a single calf, and then nose-to-nose and calf-to-calf from there, in areas where calves were concentrated in small spaces.”

In an extended type of cow-calf operation, where the cows and calves are distributed over large acreages, an outbreak may be more difficult. “You can have Mycoplasma bovis transmission and not necessarily have an outbreak,” says Rosenbusch.

In stocker operations and feedlots, Mycoplasma bovis is harbored in the respiratory tract. “The major way it’s going to be spread is through aerosols, direct nose-to-nose contact and spread of discharge from the nasal passages from one animal to another,” says Dan Grooms, DVM, PhD, Michigan State University.

“We don’t have well-documented information on respiratory transmission,” adds Rosenbusch. “What little information we have indicates that nose-to-nose contact is very important for massive transmission.”

He explains that massive transmission would occur in a stocker operation that receives calves from one or two order buyers, puts them together and then experiences a significant acute outbreak. An outbreak such as this would occur in the first two to four weeks after arrival with 80 percent to 90 percent of calves showing some kind of clinical signs and mortality up to 7 percent. (See stocker survey sidebar.)

“You can super-infect an animal that’s already infected with a strain of Mycoplasma bovis and we can isolate more than one field strain from a nose swab from a single animal,” says Rosenbusch. “I’ve never seen an animal with three strains, but I see no reason why they wouldn’t be there. I’ve seen many with one strain and a few with two strains. Even though it might be ubiquitous, having a Mycoplasma bovis circulating through a stocker operation is no guarantee that they are immune to a different strain coming in. A second strain could cause an outbreak.”

A study by Amelia Woolums, DVM, University of Georgia College of Veterinary Medicine, showed that when they did PCR recovery out of the noses of beef cow herds, they found the cows free of Mycoplasma. “This tells me that an order buyer is buying a lot of seronegative naïve calves and running them together with a few, or perhaps many, infectious calves,” explains Rosenbusch. “That leads to an explosive situation.”

Transmission can also occur through a contaminated environment. Though research information is lacking, a French study of Mycoplasma synoviae, an arthritis-causing pathogen in chickens that produces a disease similar to that of Mycoplasma bovis, showed that environmental contamination resulted in a slow, simmering and late outbreak, whereas an infectious animal in contact with naïve animals resulted in an acute outbreak. “It’s been a concern for the poultry industry for years,” says Rosenbusch. “You’re looking at a lower dose presentation coming from the environment. It could be a feeder or the watering device. An operation may have had an outbreak last year, and the environment stayed contaminated. So when cattle are brought in, they may not show anything for two months because it takes a while to develop.”

It’s not known how hardy Mycoplasma is in the environment. Though it can live almost a year when mixed with manure under wet conditions in a lab refrigerator, it’s probably not that durable in the environment. However, Rosenbusch says there could be some significant environmental presence in the field. “In a feedlot where you empty one lot and refill it with cattle in three days or a week, the survivability of Mycoplasma is something to think about.”

He has looked at nasal secretions contaminating a feedbunk or waterer – primary sources of environmental contamination – but the organism could also be brought in on workers’ boots. “It would not necessarily infect just the feedbunk or the water source or the soil,” says Rosenbusch. “You can imagine a broader dissemination for some time.”

Grooms agrees that Mycoplasma can lurk in the environment. “In Michigan, a lot of our cattle are fed under roofs where they never see sunshine. I think the chances of environmental contamination with Mycoplasma is much greater.”

Whether it can infect calves through colostrum is unclear. Rosenbusch has not been able to demonstrate Mycoplasma bovis in colostrum, but he has evidence that colostrum can transmit the pathogen. “Calves given nothing but natural colostrum still are getting Mycoplasma bovis. Even if diseased milk is kept out of the equation, they still get Mycoplasma bovis in the first day of life. It has to be the colostrum, although we cannot detect it in the lab.”

Dan Grooms, DVM, PhD says Myco- plasma can be transmitted through aerosols, direct nose-to-nose contact and spread of discharge from the nasal passages from one animal to another.

Some believe stress and weather patterns affect transmission and infection. “From the outbreaks I’ve studied with fingerprinting and with multiple outbreak isolates, the thing that strikes me most is that weather and stress seem to be very critical components of every one of these significant high-mortality outbreaks,” says Rosenbusch.

ImmunologyHow the immune system responds to Mycoplasma is only partially understood. Rosenbusch says lung lesions are developed long before antibody responses to Mycoplasma are produced. “But, you can also have an antibody response and still produce lesions. In other words, antibody responses are significant in protection, but you also need a cellular component.”

He adds that it is most likely a combination of innate and acquired immunity in the cellular component. “Dr. Simecka at the University of North Texas Health Science Center has a Mycoplasma model in mice, and they have exquisitely delicate ways of measuring and of answering this question. The innate component of the immune response is probably more important than anything else – more important than T-cells, more important than antibodies. Whatever those cells do probably drives whether the animal is going to have lesions or a high Mycoplasma content in the lung. However, that’s in the mouse and I’m not ready to totally extrapolate for the cow, but that’s the only data we have.”

The enzyme-linked immunosorbent assay (ELISA) test shows that all animals will seroconvert when infected with Mycoplasma. “In fact, it would be difficult to find seronegative animals these days because they see Mycoplasma bovis or other Mycoplasmas that look enough like Mycoplasma bovis that there’s going to be cross-reactivity,” says Rosenbusch. “Seronegatives are very hard to find.”

Rosenbusch adds that there is no question that there is a significant immune response to Mycoplasma, but it’s not protective enough to protect the animal from super infection. “Once an animal is exposed to Mycoplasma bovis and runs its clinical or subclinical course, it probably is immune to further clinical forms of Mycoplasma infection, but it is not protected from super-infection with a different strain.”

It’s unknown if there’s cross-reactivity between strains of Mycoplasma bovis. Two problem species common in cattle are M. bovirhinis and M. bovigenitalium, which are usually considered non-pathogenic. “Yet, cattle will mount seroresponses to those two organisms that will be detected as seroresponses to Mycoplasma bovis,” says Rosenbusch. “I don’t consider those animals protected, but I haven’t tested them.”

What this means for immunologic testing is that you may be confident of a seronegative result. “I would say that animal had not seen either Mycoplasma bovis or some other look-alike,” Rosenbusch explains. “But if I see a seropositive, I don’t know what that means. Many things could cause that seropositive. I’m not encouraging any lab to go into serology at this point because it would be a quagmire.”

Serology could be used to find negative animals, however. Paired serology could indicate increases in titers shortly after arrival to determine if an M. bovis-like agent was producing a seroconversion.

What’s confusing is that in the lab, M. bovis is immunosuppressive and actually kills lymphocytes, B-cells, T-cells and all subsets of T-cells. “And yet the animal in the field does not show significant immunosuppression,” says Rosenbusch. “This has been published. You can give a strain-19 Brucella vaccine and infect with M. bovis and have controls not infected with M. bovis. You’ll see a good response to the strain-19 vaccination in both groups. It doesn’t interfere with concurrent vaccination.”

Environmental contamination of Mycoplasma can be exacerbated by housing conditions such as covered feedlots in some areas of the country.

Rosenbusch compares M. bovis to bovine viral diarrhea virus (BVDV). “BVDV is an immunosuppressive agent, and when you do a necropsy, you don’t need to take a slice of the lymph nodes. You can just look at them – they are depleted. When I infect with Mycoplasma bovis, regardless of the route of administration, I always get the tonsils positive by culture. When I do immunohistochemistry and histopath on the tonsils, they are not depleted. In very young calves, I’ve seen a hint of partial depletion with a very high dose of Mycoplasma bovis. Mostly, it’s the mesenteric lymph nodes.”

Rosenbusch notes that occasionally researchers stumble onto one or two calves that are refractory to a Mycoplasma bovis challenge. “Is it genetically resistant to developing lesions? It may have very high titers of Mycoplasma bovis in the lungs, but no lesions. I do believe there are some genetically resistant animals out there.”

How long-lasting immunity to Mycoplasma is isn’t completely known. Borrowing heavily from other Mycoplasma models, Rosenbusch believes immunity following natural infection and vaccination may be only two to three months.

The interesting contrast with experiments done in Britain is that when they infected cattle with Mycoplasma bovis and then went in with a Brucella strain-19, no interaction or inhibition of the immune response occurred, notes Rosenbusch. “Yet when they did the same experiment and went in with M. dispar, they saw significant immunosuppression and reduction of the response to strain-19. Here’s a Mycoplasma that’s not highly pathogenic. Generally, there’s no mortality and it’s highly immunosuppressive. This is an area of a lot of contradiction.”

PathogenesisBeef cattle infections of Mycoplasma bovis can take many forms, from respiratory disease to otitis to arthritis. “Mycoplasma bovis strains are variable,” says Rosenbusch. “Some of them will destroy the tracheal escalator; the majority of them do not. Some go systemic and cause arthritis; many do not. Some cause a febrile reaction; others do not. From one phenotype, I can take a strain and define a set of pathogenicity markers, clinical phenotypes, for that strain.” He adds that the organism is currently being sequenced at the University of Missouri, which will provide useful genome information for one strain of Mycoplasma bovis.

Infection can cause several problems at the same time, though they may not be evident visually. An example would be a field infection where arthritis is predominant and there is no respiratory sign, but on necropsy, the lungs are infected. Others may have a complete pneumonic presentation and not a single animal has a clinical presentation of arthritis, which, according to Rosenbusch, is very common. “I’d say arthritis represents less than 20 percent of the total symptoms.”

Rosenbusch adds that all Mycoplasma bovis strains will cause mastitis. But when you get into the respiratory and systemic component, that’s where you see a difference. “If you have a strain that is not affecting the tracheal escalator, then how does it get down to the lung?” he asks. “It needs a heavy component of some other bovine respiratory disease (BRD) pathogen to make its route down.”

Some experiments Rosenbusch did with John Andrews, DVM, PhD, at Iowa State University showed that a hot strain of Hemophilus somnus introduced into the nose would induce severe pneumonia in almost 100 percent of the animals. “In animals where we had Mycoplasma bovis in the nose, a necropsy revealed Mycoplasma bovis in the lungs in four days,” says Rosenbusch. “The animal had both Hemophilus and Mycoplasma bovis in the lungs. The controls that were not infected had Mycoplasma bovis in the nose and that never got into the lungs. It was a strain that couldn’t make it down to the lung on its own, but it easily went down once the tracheal defenses were destroyed.”

This information is from a Bovine Veterinarian Mycoplasma bovis roundtable sponsored by Boehringer Ingelheim and moderated by Bob Glock, DVM, PhD.

A 2001 study by Kansas State University of stocker/backgrounders in Kansas asked producers about the following clinical syndrome: About two weeks after arrival, calves pulled for pneumonia were nonresponsive after two antibiotics were tried. About three weeks after arrival, arthritic calves were pulled, with progressive conditions.

Some of the key findings were:

The syndrome was reported across all sizes of operations, especially larger operations.

The syndrome occurred in all weights of cattle but more often lighter-weight cattle.

Outbreaks were more likely to occur when more loads were increased during the winter.

The syndrome was reported in cattle from all areas, but less likely to be found in home-raised calves or calves from the western U.S.

The likelihood of a problem increased as cattle were received from an increasing number of states.

Increased stress from castration or dehorning (upon arrival or delayed) may have increased the likelihood of having a problem.